Doped halide scintillators

ABSTRACT

The present invention provides for a composition comprising an inorganic scintillator comprising a doped halide, useful for detecting nuclear material.

RELATED PATENT APPLICATIONS

This application claims priority to U.S. Provisional Patent Application Ser. No. 61/813,130, filed Apr. 17, 2013, which is hereby incorporated by reference in its entirety.

STATEMENT OF GOVERNMENTAL SUPPORT

The invention was made with government support under Grant No. HSHQDC-07-X-00170 awarded by the U.S. Department of Homeland Security, and Contract No. DE-AC02-05CH11231 awarded by the U.S. Department of Energy. The government has certain rights in the invention.

FIELD OF THE INVENTION

The present invention is in the field of inorganic crystals with scintillation properties useful as gamma-ray detectors.

BACKGROUND OF THE INVENTION

The need for radiation detecting materials has been at the forefront of materials research in recent years due to applications in national security, medical imaging, X-ray detection, oil well logging, and high-energy physics. Essential qualities that a scintillator must possess are high light yields, fast luminescence decay (below 1000 ns), good stopping power, high density, good energy resolution, ease of growth, proportionality, and stability under ambient conditions. La_(x)Br₃:Ce_(1-x) (E. V. D. van Loef et al, Appl. Phys. Lett., 2001, 79, 1573) and Sr_(x)I₂:Eu_(1-x)(N. Cherepy et al, Appl. Phys. Lett. 2007, 92, 083508) are present day benchmark materials that satisfy some of the desired criteria, but their application is limited due to the extreme hygroscopic nature.

CsBa₂I₅:Eu has been previously disclosed (Bourret-Courchesne et al., Eu²⁺-doped Ba₂CsI₅, a new high-performance scintillator, Nucl. Instru. Met. Phys. Res. A, 612:138-142, 2009).

SUMMARY OF THE INVENTION

The present invention provides for a composition comprising an inorganic scintillator comprising a doped halide, useful for detecting nuclear material.

The present invention provides for an inorganic scintillator having the formula: A′_(y)B′_(z)X₅:M′_(x)  (I); wherein X is a halogen or a mixture of different halogen elements; M′ is an element with a valence of 1+; A′ is an alkali metal with a valence of 1+; B′ is an alkaline earth metal with a valence of 2+; x has a value having the range 0<x<5; and y+2z+x=5. In some embodiments, the stoichiometry of A′:B′ is 1:2.

The present invention provides for an inorganic scintillator having the formula: A′_(y)A″_(z)C′_(w)X₆:M′_(x)  (II); wherein X is a halogen or a mixture of different halogen elements; M′ is an element with a valence of 1+; A′ is a first alkali metal with a valence of 1+; A″ is a second alkali metal with a valence of 1+, wherein no two of A′, A″, and M′ are the same element; C′ is an element with a valence of 3+; x has a value having the range 0<x<6; and y+z+3w+x=6. In some embodiments, the stoichiometry of A′+A″:C′ is 3:1. In some embodiments, the stoichiometry of A′:A″:C′ is 1:2:1.

The present invention provides for an inorganic scintillator having the formula: A′_(y)B′_(z)C′_(w)X₆:M′_(x)  (III); wherein X is a halogen or a mixture of different halogen elements; M′ is an element with a valence of 1+; A′ is an alkali metal with a valence of 1+, wherein A′ and M′ are not the same element; B′ is an alkaline earth metal with a valence of 2+; C′ is an element with a valence of 3+; x has a value having the range 0<x<6; and y+2z+3w+x=6. In some embodiments, the stoichiometry of A′:B′:C′ is 1:1:1.

The present invention provides for an inorganic scintillator having the formula: A′_(y)C′_(w)X₆:M′_(x)  (IV); wherein X is a halogen or a mixture of different halogen elements; M′ is an element with a valence of 1+; A′ is an alkali metal with a valence of 1+, wherein A′ and M′ are not the same element; C′ is an element with a valence of 3+; x has a value having the range 0<x<6; and y+3w+x=6. In some embodiments, the stoichiometry of A′:C′ is 3:1.

No two A′, A″, B′, C′, and M′ are the same element. In some embodiments, X is F, Br, Cl, I, or a mixture thereof. In some embodiments, A′, A″ are each independently Li, Na, K, Rb, Cs, or Fr. In some embodiments, B′ is Be, Mg, Ca, Sr, Ba, or Ra. In some embodiments, C′ is a metal or transition metal with a valence of 3+. In some embodiments, C′ is a lanthanide with a valence of 3+. In some embodiments, C′ is a Group 13 post-transition metal with a valence of 3+ (such as Al, Ga, In, or Tl). In some embodiments, M′ is a Group 13 post-transition metal with a valence of 1+ (such as Al, Ga, In, or Tl) or an alkali metal (such as Li, Na, K, Rb, Cs, or Fr). In some embodiments, M′ is In, Tl, or Na. In some embodiments, A′ is Cs, B′ is Ba, and M′ is In, Tl, or Na. In some embodiments, A′ is Cs, B′ is Ba, X is I, and M′ is In, Tl, or Na.

In some embodiments, x has a value having the range 0<x≤4. In some embodiments, x has a value having the range 0<x≤3. In some embodiments, x has a value having the range 0<x≤2. In some embodiments, x has a value having the range 0<x≤1. In some embodiments, x has a value having the range 0<x≤0.5. In some embodiments, x has a value having the range 0<x≤0.25. In some embodiments, x has a value having the range 0<x≤0.1.

In some embodiments of the invention, x has a value having the range 0<x≤2, 0<x≤1, 0.001≤x≤2, 0.001≤x≤1, 0.001≤x≤0.5, 0.01≤x≤2, 0.01≤x≤1, 0.01≤x≤0.5, 0.1≤x≤2, 0.1≤x≤1, or 0.1≤x≤0.5. In some embodiments of the invention, where M′ is indicated as a molar percentage of the anions, suitable amounts of M′ range from over 0% to 10%, over 0% to 7%, over 0% to 5%, 1% to 10%, 1% to 5%, 1% to 4%, or 2% to 3%.

In some embodiments of the invention, the inorganic scintillator is a single crystal having at least one dimension of a length of at least 1 mm, at least 5 mm, at least 1 cm, or at least 3 cm, or a length at least sufficient to stop or absorb gamma-radiation.

The present invention provides for an inorganic scintillator described and/or having one or more of the properties described in Example 1.

The present invention also provides for a composition comprising essentially of a mixture of halide salts (comprising A′ halide, M′ halide, optionally A″ halide, optionally B′ halide, and optionally C′ halide) useful for producing the inorganic scintillator of the present invention, wherein each elements relative to each other within the composition have a stoichiometry essentially equivalent to the stoichiometry of the elements in the compounds of formulae (I)-(IV), or any other formulae, as described herein.

The halide salts can be powdered crystals. The halide salts are essentially pure. Such halide salts are commercially available. Such halide salts include, but are not limited to, CsX, BaX₂, TlX, InX, NaX, and the like.

The present invention further provides for a method for producing the composition comprising an inorganic scintillator as described herein comprising: (a) providing a composition comprising essentially of a mixture of halide salts described herein useful for producing the inorganic scintillator as described herein, (b) heating the mixture so that the halide salts start to react, and (c) cooling the mixture to room temperature such that the composition comprising an inorganic scintillator is formed.

The invention provides for a device comprising a composition comprising an inorganic scintillator of the present invention and a photodetector. The device is useful for the detection of an ionizing radiation, such as gamma radiation. The device is useful for industrial, medical, protective and defensive purpose or in the oil and nuclear industry.

In some embodiments of the invention, the device is a gamma ray (or like radiation) detector which comprises a single crystal of the inorganic scintillator of the present invention. When assembled in a complete detector, the scintillator crystal is optically coupled, either directly or through a suitable light path, to the photosensitive surface of a photodetector for generation of an electrical signal in response to the emission of a light pulse by the scintillator. The inorganic scintillator of the invention possesses certain important characteristics, most notably high light output, very short decay time and high detection efficiency, that make it superior to prior scintillators as a gamma ray or like radiation detector, in particular for homeland security applications, such as nuclear material detection.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing aspects and others will be readily appreciated by the skilled artisan from the following description of illustrative embodiments when read in conjunction with the accompanying drawings.

FIG. 1 shows (A) the room temperature X-ray excited emissions for CsBa₂I₅ containing different Tl concentrations, and (B) the room temperature optical excitation (dashed curve) and emission spectra (solid curves) for CsBa₂I₅:Tl (0.25%). The emission curves are recorded on excitation at 240, 265, and 280 nm.

FIG. 2 shows the estimated luminosity for CsBa₂I₅ containing different Tl concentrations. Error bars represent statistical variations in the measurements.

FIG. 3 is a diagrammatic view of one embodiment of a scintillation detector in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Before the present invention is described, it is to be understood that this invention is not limited to particular embodiments described, as such may, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting, since the scope of the present invention will be limited only by the appended claims.

Where a range of values is provided, it is understood that each intervening value, to the tenth of the unit of the lower limit unless the context clearly dictates otherwise, between the upper and lower limits of that range is also specifically disclosed. Each smaller range between any stated value or intervening value in a stated range and any other stated or intervening value in that stated range is encompassed within the invention. The upper and lower limits of these smaller ranges may independently be included or excluded in the range, and each range where either, neither or both limits are included in the smaller ranges is also encompassed within the invention, subject to any specifically excluded limit in the stated range. Where the stated range includes one or both of the limits, ranges excluding either or both of those included limits are also included in the invention.

Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Although any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, the preferred methods and materials are now described. All publications mentioned herein are incorporated herein by reference to disclose and describe the methods and/or materials in connection with which the publications are cited.

As used in the specification and the appended claims, the singular forms “a”, “an”, and “the” include plural references unless the context clearly dictates otherwise. Thus, for example, reference to a “crystal” includes a single crystal as well as a plurality of crystals.

The terms “optional” or “optionally” as used herein mean that the subsequently described feature or structure may or may not be present, or that the subsequently described event or circumstance may or may not occur, and that the description includes instances where a particular feature or structure is present and instances where the feature or structure is absent, or instances where the event or circumstance occurs and instances where it does not.

These and other objects, advantages, and features of the invention will become apparent to those persons skilled in the art upon reading the details of the invention as more fully described below.

The Inorganic Scintillators

Useful qualities for the inorganic scintillators of the present invention are high light yields, fast luminescence decay (such as, equal to or less 1000 ns), good stopping power, high density, good energy resolution, ease of growth, and stability under ambient conditions.

In some embodiments of the invention, the mixed halide combination is: (a) Cl and Br, (b) Cl and I, or (c) Br and I.

The inorganic scintillator can be in a polycrystalline powder or a single crystal form. The crystal can be any size with an average volume of at least 0.001 mm³, at least 1 mm³, at least 5 mm³, at least 10 mm³, at least 100 mm³, at least 3 cm³, at least 1 cm³, or at least 10 cm³. The crystal can be any size with at least one dimension of the crystal having a length of at least 100 μm, at least 1 mm, at least 2 mm, at least 5 mm, at least 1 cm, at least 3 cm, at least 5 cm, or at least 10 cm. In some embodiments of the invention, the crystal has at least one dimension having a length that is of sufficient length, or depth, to stop or absorb gamma-radiation in order to electronically detect the gamma-radiation.

The inorganic scintillators of the present invention are useful as they are scintillators and they produce a useful bright and fast scintillation in response to irradiation by short-wavelength high energy light, such as x-ray or gamma rays. The crystals of the inorganic scintillator also have the added advantage of having the property of readily growing into crystals. Large size crystals can be grown by the following technique: Bridgman growth and related techniques, Czochralski growth and related techniques, the traveling heater method and related techniques.

The inorganic scintillators of the present invention have one or more of the following advantageous aspects: high light yields, good energy resolution, proportional response to gamma rays with different energies, ease of growth, and stability under ambient conditions.

Characterization of the Inorganic Scintillators

The crystals of the invention can be characterized using a variety of methods. The crystals can be characterized regarding X-ray diffractometry, X-ray luminescence spectra, X-ray fluorescence for concentration of activators, and/or pulsed X-ray time response. X-ray diffractometry determines the composition of crystalline solids, such as crystalline phase identification. X-ray luminescence spectra determine the spectra components. Pulsed X-ray time response determines luminosity, decay times, and fractions. X-ray luminescence is used to determine the relative luminosity of a crystal. An X-ray excited emission spectra is obtained of a crystal by irradiating the crystal with an X-ray and collecting the emission light, such as at 90°, by a CCD detector.

In some embodiments of the invention, the luminosity of the inorganic scintillator is more than the luminosity of yttrium aluminium perovskite (YAP) and/or bismuth germanate (BGO). In further embodiments of the invention, the luminosity of the inorganic scintillators is more than double the luminosity of YAP and/or BGO.

In some embodiments of the invention, the single crystal inorganic scintillators, such as CsBa₂I₅:Tl, have a luminescence output equal to or more than 20,000 photons/MeV, and a decay of equal to or more than 30% of the emitted light in a period equal to or less than 500 ns. In some embodiments, the luminescence output is equal to or more than 30,000, 50,000, 70,000, or 90,000 photons/MeV. In some embodiments, the decay is equal to or more than 30 or 40% of the emitted light in a period equal to or less than 450, 400, or 350 ns.

Preparation of the Inorganic Scintillators

The inorganic scintillators of the invention can be prepared using a variety of methods. For example, the crystals useful for fabrication of luminescent screens can be prepared by a solid-state reaction aided, or optionally not aided, by a flux of halides as described herein. In some embodiments, the single crystals are prepared by providing a composition comprising essentially of a mixture of halide salts useful for producing the inorganic scintillator as described herein. The mixture is heated to a temperature of up to about 550-900° C. using a simple programmable furnace to produce a reactive molten mixture. The reaction is maintained at temperature for the mixture to fully react and produce the desired melt. The resultant molten product of reaction is then cooled slowly at about 2 to 5° C./minute.

A particular method of preparing the inorganic scintillator of the invention is as follows: Bridgman growth and related techniques, Czochralski growth and related techniques, the traveling heater method and related techniques. These methods can be used to produce the inorganic scintillator as single crystals on a one-by-one basis.

The Bridgman growth technique is a directional solidification process. The technique involves using an ampoule containing a melt which moves through an axial temperature gradient in a furnace. Single crystals can be grown using either seeded or unseeded ampoules. The Bridgman growth technique is taught in Robertson J. M., 1986, Crystal growth of ceramics: Bridgman-Stockbarger method in Bever: 1986 “Encyclopedia of Materials Science and Engineering” Pergamon, Oxford pp. 963-964, which is incorporated by reference.

The Czochralski growth technique comprises a process of obtaining single-crystals in which a single crystal material is pulled out of the melt in which a single-crystal seed is immersed and then slowly withdrawn; desired optical properties and doping level is accomplished by adding dopants to the melt. The Czochralski growth technique is taught in J. Czochralski, “Ein neues Verfahren zur Messung der Kristallisationsgeschwindigheit der Metalle” [A new method for the measurement of the crystallization rate of metals], Z Phys. Chemie 92 (1918) 219-221, which is incorporated by reference. The method is well-know to those skilled in the art in producing a wide variety of compounds, including semiconductors and scintillator materials (such as LaBr₃:Ce).

The traveling heater method is described in Triboulet, Prog. Cryst. Gr. Char. Mater., 128, 85 (1994) and Funaki et al., Nucl. Instr. And Methods, A 436 (1999), which are incorporated in their entireties by reference.

A particular method of preparing inorganic scintillators of the invention is the ceramic method which comprises the following steps: The reactant mixture is placed in a container, such as a glove box, filled with one or more inert gas, such as nitrogen gas. The container is under a very dry condition. The dry condition is required due to the hygroscopic nature of the halides within the reactant mixture. The two or more powder reactants are ground together, such as with a mortar and pestle, for a sufficient period, such as about 10 minutes, to produce a reactant mixture. When Ln halide salt is added to the powder reactants for grinding, a suitable organic solvent or solution can be further added, and grinding can take place until the mixture appears dry. The reactant mixture is sintered under high temperature and pressure.

In some embodiment of the invention, the single crystals of the inorganic scintillator can be grown by melting and re-solidifying the pre-synthesized compounds in powder form, such as described herein, or directly from melting the mixtures of the halides salts and lanthanide halides used as activators. To grow best performing crystals the starting compounds might need to be purified further by zone refining.

Growing the single crystal involves loading the mixtures, such as described herein, in a quartz ampoule in a dry environment and sealing the ampoule using a high temperature torch, maintaining the dry environment at a reduced pressure, in the ampoule. The ampoule is then placed in a furnace. The growth of the crystal can be performed by a variation of the known vertical “Bridgman” technique. The compound is melted, let to homogenized at a temperature above the melting point and the compound is solidified in a directional manner in a temperature gradient. The ampoule is shaped to provide a nucleation site at the bottom (conical shape). The solidification front moves upward. Horizontal configurations and other growth techniques such as Czochralski (may need to pressurized the growth chamber) could be used.

In some embodiments of the invention, the method for producing the composition comprising the inorganic scintillator of the present invention comprises: (a) providing a sealed container containing the composition comprising essentially of a mixture of halide salts useful for producing the inorganic scintillator of the present invention, (b) heating the container sufficiently to produce a melted mixture, and (c) solidifying or growing a crystal from the melted mixture, such that the composition comprising the inorganic scintillator of the present invention is produced.

In some embodiments of the invention, the method for producing the composition comprising the inorganic scintillator of the present invention comprises: (a) providing the composition comprising essentially of a mixture of halide salts, (b) loading the halide salts in a suitable container, (c) sealing the container, (d) heating the container sufficiently to produce a melted mixture, and (e) solidifying or growing a crystal from the melted mixture, such that the composition comprising the inorganic scintillator of the present invention is produced.

In some embodiments, the container is a quartz container. In some embodiments, the sealed container is an ampoule. In some embodiments, the heating takes place in a furnace. The mixture is heated to a suitable temperature to melt the halides in the mixture. One skilled in the art can easily determine a temperature or a range of temperatures suitable for melting the mixture of halides, such as, including but not limited to, a temperature of about 650-750° C., 600-850° C., 860° C., 650-740° C., 600° C.-860° C., or 550-740° C. The furnace can be a Bridgman-type or float-zone-type (minor-furnace where heat s supplied by halogen lamps, or induction heated furnace). When using a Bridgman configuration, the crystal is solidified from the melt directionally. When using the float-zone configuration, the crystal is solidified from a narrow molten zone of a pre-reacted charge. In both cases the growth rate of the crystal can be within a thermal gradient across the solid/liquid interface. The ratio of gradient to growth rate determines the stability of the interface. The growth rate can be decreased if the thermal gradient is increased. Typical thermal gradient can be more than 1° C./cm. Once all solidified, the crystal is cooled slowly. The cooling rate can be in the range from less than 1° C./hr to more than 20° C./hr.

The resulting crystals are then characterized by the methods described herein. The resulting crystals also have properties similar to those described herein.

Application of the Inorganic Scintillators

The present invention provides for a gamma ray or x-ray detector, comprising: a scintillator composed of a transparent single crystal of the inorganic scintillator of the present invention, and a photodetector optically coupled to the scintillator for producing an electrical signal in response to the emission of a light pulse by the scintillator.

The inorganic scintillators of this invention have many advantages over other known crystals. The inorganic scintillators produce a luminescence in response irradiation, such as irradiation by alpha-, beta-, or gamma-radiation, that is brighter and faster than known and commercially used scintillators. The scintillating crystals have a number of applications as detectors, such as in the detection of gamma-ray, which has use in national security, such as for detection of nuclear materials, and medical imaging applications.

The invention is useful for the detection of ionizing radiation. Applications include medical imaging, nuclear physics, nondestructive evaluation, treaty verification and safeguards, environmental monitoring, and geological exploration. This will be a major improvement, providing much finer resolution, higher maximum event rates, and clearer images.

Also, activated inorganic scintillator crystals of the present invention can be useful in positron emission tomography (PET).

The invention also relates to the use of the scintillating material above as a component of a detector for detecting radiation in particular by gamma rays and/or X-rays. Such a detector especially comprises a photodetector optically coupled to the scintillator in order to produce an electrical signal in response to the emission of a light pulse produced by the scintillator. The photodetector of the detector may in particular be a photomultiplier, photodiode, or CCD sensor.

A particular use of this type of detector relates to the measurement of gamma or x-ray radiation, such a system is also capable of detecting alpha and beta radiation and electrons. The invention also relates to the use of the above detector in nuclear medicine apparatuses, especially gamma cameras of the Anger type and positron emission tomography scanners (see, for example C. W. E. Van Eijk, “Inorganic Scintillator for Medical Imaging”, International Seminar New types of Detectors, 15 19 May 1995—Archamp, France. Published in “Physica Medica”, Vol. XII, supplement 1, June 96; hereby incorporated by reference).

In another particular use, the invention relates to the use of the above detector in detection apparatuses for oil drilling (see, for example “Applications of scintillation counting and analysis”, in “Photomultiplier tube, principle and application”, chapter 7, Philips; hereby incorporated by reference).

One embodiment of the invention is shown in FIG. 3 which shows a gamma ray detector. The detector can be one as described in U.S. Pat. No. 4,958,080, hereby incorporated by reference. It will be understood, of course, that the utility of the novel single crystal inorganic scintillator of the invention is not limited to the detection of gamma radiation but that it has general application to the detection of other types of like radiation, e.g. X-rays, cosmic rays, and energetic particles.

In FIG. 3, a single crystal inorganic scintillator 10 is shown encased within the housing 12 of a gamma ray detector. One face 14 of the scintillator is placed in optical contact with the photosensitive surface of a photomultiplier tube 16. Alternatively, the light pulses could be coupled to the photomultiplier via light guides or fibers, lenses, minors, or the like. The photomultiplier can be replaced by any suitable photodetector such as a photodiode, microchannel plate, etc. In order to direct as much of each light flash to the photomultiplier as possible, the other faces 18 of the inorganic scintillator are preferably surrounded or covered with a reflective material, e.g. Teflon tape, magnesium oxide powder, aluminum foil, or titanium dioxide paint. Light pulses emitted by the crystal inorganic scintillator upon the incidence of radiation are intercepted, either directly or upon reflection from the surfaces 18, by the photomultiplier, which generates electrical pulses or signals in response to the light pulses. These electrical output pulses are typically first amplified and then subsequently processed as desired, e.g. in a pulse height amplifier, to obtain the parameters of interest regarding the detected radiation. The photomultiplier is also connected to a high voltage power supply, as indicated in FIG. 3. Other than the inorganic scintillator, all of the components and materials referred to in connection with FIG. 3 are conventional, and thus need not be described in detail.

It is to be understood that, while the invention has been described in conjunction with the preferred specific embodiments thereof, the foregoing description is intended to illustrate and not limit the scope of the invention. Other aspects, advantages, and modifications within the scope of the invention will be apparent to those skilled in the art to which the invention pertains.

All patents, patent applications, and publications mentioned herein are hereby incorporated by reference in their entireties.

The invention having been described, the following examples are offered to illustrate the subject invention by way of illustration, not by way of limitation.

EXAMPLE 1 Characterization of CsBa₂I₅:Tl⁺

Powders of the composition CsBa₂I₅:Tl⁺ are obtained by a solid state route using commercial chemicals without further purification. The compound is subsequently grown as a single crystal using the Bridgman technique. Crystals (with maximum dimensions<2 mm) are synthesized from the melt sealed quartz ampoules. Based on size and charge, Tl⁺ (ionic radius 1.55 Å 0 in the lattice. The Tl concentration is varied between 0 and 1 mol %. Powdered X-ray diffraction is carried out to ascertain the purity of the samples. Emission/decay response is observed on X-ray and optical excitation.

FIG. 1(A) shows the X-ray excited emission curves for undoped CsBa₂I₅ as well as samples containing 0.05 to 1% Tl⁺. The undoped sample (dotted line) shows a broad emission between 350 and 550 nm. Upon Tl addition, it is observed a broad emission between 420 and 650 nm. The emission is a superposition of multiple peaks. Further increase in the Tl concentration results in an increase in the peak intensity. The maximum intensity is obtained on activation with 0.25 mol% of Tl⁺.

FIG. 1(B) shows the room temperature optical excitation and emission curves for CsBa₂I₅:Tl⁺ (0.25%). Upon excitation between 240 and 300 nm, the emission spectra shows a broad peak between 420 and 650 nm. The emission is identical to that obtained on X-ray excitation. There is no overlap between excitation and emission curves: it indicates that there is no self-absorption that could be detrimental to the performance of the material.

Table 1 shows the results obtained for the luminescence decay components for the samples upon X-ray excitation. All Tl-doped samples show a fast decay component between 360 and 500 ns, accounting for 30-40% of the total light. These decay times observed are faster than those observed using Eu activation in the same crystals.

Light yields are estimated from the pulsed X-ray decay measurements by summing all detected photons. Relative light yields of each sample are obtained by comparison to a CsBa₂I₅:Eu single crystal with known light yield (100,000 photons/MeV) measured under identical conditions. The luminosities in FIG. 2 correspond to statistical variations obtained in the measurements. CsBa₂I₅ containing 0.25% Tl dopant shows the highest light output, ˜72,400 photons/MeV.

TABLE 1 Variation of the luminescence decay components with Tl concentration for CsBa₂I₅ measured at room temperature under X-ray excitation Stop Decay components (ns) Tl % luminosity τ₁ τ₂ τ₃ τ₄ τ₅ τ₆ Slow 0 5490 22 65 118 278 (12%) 808 (15%) 3827 (14%) 14% (9%) (19%) (16%) 0.05 21,400 247 (8%)  466 (41%) 3155 (13%) 38% 0.1 27,100 158 (4%)  383 (38%) 2167 (11%) 47% 0.25 72,400 364 (42%) 920 (23%) 35% 0.5 54,600 208 (7%)  391 (42%) 2119 (8%)  43% 1 35,100 191 (10%) 410 (32%) 2810 (11%) 45%

These inorganic scintillator crystals are useful for national security purposes, such as detecting nuclear material.

While the present invention has been described with reference to the specific embodiments thereof, it should be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the true spirit and scope of the invention. In addition, many modifications may be made to adapt a particular situation, material, composition of matter, process, process step or steps, to the objective, spirit and scope of the present invention. All such modifications are intended to be within the scope of the claims appended hereto. 

What is claimed is:
 1. An inorganic scintillator having the formula: A′_(y) B′_(z) X₅:M′_(x)(I); wherein X is a halogen or a mixture of different halogen elements; M′ is an element with a valence of 1+; M′ is a Group 13 post-transition metal; A′ is an alkali metal with a valence of 1+; B′ is an alkaline earth metal with a valence of 2+; A′ and M′ are not the same element; A′ and B′ are present in the inorganic scintillator; x has a value having the range 0<x<5; and y +2z +x =5; wherein the inorganic scintillator is a single crystal having at least one dimension of the crystal having a length of at least 1 mm.
 2. The inorganic scintillator of claim 1, wherein the ratio of the stoichiometry of A′:B′ is 1:2.
 3. A gamma ray detector comprising the inorganic scintillator of claim
 1. 4. The inorganic scintillator of claim 1, wherein M′ is Al, Ga, In, or TI.
 5. The inorganic scintillator of claim 4, wherein M′ is TI.
 6. The inorganic scintillator of claim 1, wherein the inorganic scintillator is a single crystal having at least one dimension of the crystal having a length of at least 1 cm. 